Client–server systems are today most frequently implemented by (and often identified with) the request–response model: a client sends a request to the server, which performs some action and sends a response back to the client, typically with a result or acknowledgement. Designating a computer as "server-class hardware" implies that it is specialized for running servers on it. This often implies that it is more powerful and reliable than standard personal computers, but alternatively, large computing clusters may be composed of many relatively simple, replaceable server components.

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The use of the word server in computing comes from queueing theory,[3] where it dates to the mid 20th century, being notably used in Kendall (1953) (along with "service"), the paper that introduced Kendall's notation. In earlier papers, such as the Erlang (1909), more concrete terms such as "[telephone] operators" are used.

In computing, "server" dates at least to RFC 5 (1969),[4] one of the earliest documents describing ARPANET (the predecessor of Internet), and is contrasted with "user", distinguishing two types of host: "server-host" and "user-host". The use of "serving" also dates to early documents, such as RFC 4,[5] contrasting "serving-host" with "using-host".

The Jargon File defines "server" in the common sense of a process performing service for requests, usually remote, with the 1981 (1.1.0) version reading:

SERVER n. A kind of DAEMON which performs a service for the requester, which often runs on a computer other than the one on which the server runs.

A network based on the client–server model where multiple individual clients request services and resources from centralized servers

Strictly speaking, the term server refers to a computer program or process (running program). Through metonymy, it refers to a device used for (or a device dedicated to) running one or several server programs. On a network, such a device is called a host. In addition to server, the words serve and service (as noun and as verb) are frequently used, though servicer and servant are not.[a] The word service (noun) may refer to either the abstract form of functionality, e.g. Web service. Alternatively, it may refer to a computer program that turns a computer into a server, e.g. Windows service. Originally used as "servers serve users" (and "users use servers"), in the sense of "obey", today one often says that "servers serve data", in the same sense as "give". For instance, web servers "serve [up] web pages to users" or "service their requests".

The server is part of the client–server model; in this model, a server serves data for clients. The nature of communication between a client and server is request and response. This is in contrast with peer-to-peer model in which the relationship is on-demand reciprocation. In principle, any computerized process that can be used or called by another process (particularly remotely, particularly to share a resource) is a server, and the calling process or processes is a client. Thus any general purpose computer connected to a network can host servers. For example, if files on a device are shared by some process, that process is a file server. Similarly, web server software can run on any capable computer, and so a laptop or a personal computer can host a web server.

While request–response is the most common client–server design, there are others, such as the publish–subscribe pattern. In the publish–subscribe pattern, clients register with a pub–sub server, subscribing to specified types of messages; this initial registration may be done by request–response. Thereafter, the pub–sub server forwards matching messages to the clients without any further requests: the server pushes messages to the client, rather than the client pulling messages from the server as in request–response.[6]

When referring to hardware, the word server typically designates computer models specialized for their role. In general, a server performs its role better than a generic personal computer.

The purpose of a server is to share data as well as to share resources and distribute work. A server computer can serve its own computer programs as well; depending on the scenario, this could be part of a quid pro quo transaction, or simply a technical possibility. The following table shows several scenarios in which a server is used.

Hosts web apps (computer programs that run inside a web browser) allowing users in the network to run and use them, without having to install a copy on their own computers. Unlike what the name might imply, these servers need not be part of the World Wide Web; any local network would do.

Maintains an index or table of contents of information that can be found across a large distributed network, such as computers, users, files shared on file servers, and web apps. Directory servers and name servers are examples of catalog servers.

Any computer program that needs to find something on the network, such a Domain member attempting to log in, an email client looking for an email address, or a user looking for a file

Maintains an environment needed for one communication endpoint (user or devices) to find other endpoints and communicate with them. It may or may not include a directory of communication endpoints and a presence detection service, depending on the openness and security parameters of the network

Shares digital video or digital audio over a network through media streaming (transmitting content in a way that portions received can be watched or listened to as they arrive, as opposed to downloading an entire file and then using it)

User-attended personal computers equipped with a monitor and a speaker

Acts as an intermediary between a client and a server, accepting incoming traffic from the client and sending it to the server. Reasons for doing so includes content control and filtering, improving traffic performance, preventing unauthorized network access or simply routing the traffic over a large and complex network.

Any networked computer

Virtual server

Shares hardware and software resources with other virtual servers. It exists only as defined within specialized software called hypervisor. The hypervisor presents virtual hardware to the server as if it were real physical hardware[7]. Server virtualization allows for a more efficient infrastructure.

Almost the entire structure of the Internet is based upon a client–server model. High-level root nameservers, DNS, and routers direct the traffic on the internet. There are millions of servers connected to the Internet, running continuously throughout the world[8] and virtually every action taken by an ordinary Internet user requires one or more interactions with one or more server. There are exceptions that do not use dedicated servers; for example peer-to-peer file sharing, some implementations of telephony (e.g. pre-Microsoft Skype).

These types of servers are often housed in dedicated data centers. These will normally have very stable power and Internet and increased security. Noise is also less of a concern, but power consumption and heat output can be a serious issue. Server rooms are equipped with air conditioning devices.

A server farm or server cluster is a collection of computer servers maintained by an organization to supply server functionality far beyond the capability of a single device. Modern data centers are now often built of very large clusters of much simpler servers,[10] and there is a collaborative effort, Open Compute Project around this concept.

In 2010, data centers (servers, cooling, and other electrical infrastructure) were responsible for 1.1-1.5% of electrical energy consumption worldwide and 1.7-2.2% in the United States.[13] One estimate is that total energy consumption for information and communications technology saves more than 5 times its carbon footprint[14] in the rest of the economy by enabling efficiency.